Numerous solar heat collecting systems in a variety of configurations are commercially produced, and marketed. Most of which, are of non passive design. That is to say, they rely on a motor-driven pump for the circulation of the system confined heat-transfer fluid. The collection of a given amount of solar heat in B.T.U. is relatively small. In most cases of the non passive design, the power consumed by the motor-driven pump may well exceed the equivalent in collected solar heat. Hence, the employment of a fluid pump utilizing the available temperature differences within the solar heat collecting system instead of electric power, will significantly advance the state of art. The solar heat collecting system described herein employs in combination, a scheme for pumping fluid within a closed circuit, shown in our previous patents having the U.S. Pat. Nos. 4,030,303 filed Oct. 14, 1975, and the 4,041,710 filed Sept. 9, 1976.